The invention relates to electronic semiconductor devices, and, more particularly, to fabrication methods for such devices.
Semiconductor integrated circuits with high device density require minimum size structures such as short gates and shallow source/drain junctions for field effect transistors (xe2x80x9cFETxe2x80x9ds), small area emitters for bipolar transistors, and narrow interconnect lines between devices. Short gates and narrow interconnect lines lead to higher resistance for the typical polysilicon gates and first level interconnect lines, and this increases RC time delays and slows circuit operation.
One approach to reduce the resistance of polysilicon gates and interconnect lines uses a self-aligned metal silicide on top of the polysilicon; the metal may be refractory metals such as titanium, cobalt, and nickel. The silicidation process first deposits a blanket film of metal and then reacts the metal with any underlying silicon but not with underlying insulator such as silicon oxide (a nitrogen atmosphere may be used to simultaneously form the metal nitride to inhibit silicon diffusion). Lastly, the process removes the metal (plus any metal nitride) which did not become silicide. For example, U.S. Pat. No. 4,821,085 discloses titanium metal and reaction in a nitrogen atmosphere which forms titanium nitride where silicide does not form. The removal of metal (and metal nitride) in this self-aligned silicide process requires a high selectivity with respect to removal of the metal silicide, otherwise the metal silicide will be thinned and lose the advantages of siliciding. For example, the stripping of titanium nitride with the standard basic SC1 solution (12.5% ammonium hydroxide solution, 12.5% hydrogen peroxide solution, and 75% water) at room temperature has a selectivity with respect to titanium silicide of roughly 7.8 to 1. This selectively presents problems in the known silicide processes.
The present invention provides a highly selective strip of titanium nitride and titanium metal with respect to titanium silicide by the use of a basic solution with a large hydrogen peroxide to ammonium hydroxide ratio.
Further, the present invention provides a two step strip of metal and metal nitride while leaving metal silicide by a near perfect selectivity strip of the metal (including cobalt, nickel, and titanium) and metal nitrides with respect to the corresponding metal silicide followed by a brief etching of the metal slicide.
Preferred embodiments use a first acidic stripping of titanium, titanium nitride, cobalt, and nickel, such as with standard SC2 (solution of hydrochloric acid and hydrogen peroxide in water) or ozonated sulfuric acid to provide selectivity of greater than 100 to 1 with respect to the corresponding silicide, plua a short second etch of the silicide. The second step removes unwanted silicide which has formed on insulators.
A silicide phase conversion step may be inserted between the metal/nitride and silicide etches.
Advantages include simplicity of the removal of metal/nitride with high selectivity to the silicide for titanium, cobalt, and nickel.